Whelp, I got repel dark, so now all I need to do is get Victory Cry to make him even more broken beyond belief(I could also get Arms Master, but I don't think I need to make the game even more easier than extremely very easy).
Is that the King trick? If so, I think they made the king only able to summon one shadow once per turn. If there's another trick, I don't know since it's easy to get money/level-up. I had over 2mil(might have been 3) before finishing my first playthrough.
Also if you go for the true ending, I recommend getting him since the secret dungeon is well... it's not hard, but it will require you to save alive.